In recent years, water levels in aquifers have been dropping steadily and often dramatically. In many areas, annual drops in the water level have been 20-40 feet on average. Water wells typically are lined with casings having screens or other perforated areas located below the water level in the aquifers. This allows the surrounding water to flow into the well and rise in the casing to the static aquifer level. A pump is installed in the well and operates to pump water through a column pipe which extends to the surface.
In conventional practice, the pump intake is normally located above the level of the screen, and the pump normally draws down the water level inside the casing only to a level which is above the top of the screen. However, when the aquifer level drops significantly, it is often necessary to lower the pump intake to a level below the top of the screen. Because the pump normally draws the water level down inside of the well faster than water can flow into the well, the water level in this situation can be drawn down below the top screen/perforations.
This creates a number of problems, principally caused by water falling inside of the casing as it enters through the upper perforations. This falling water entrains air and can result in unduly aerated water being pumped from the well. The aerobic conditions that are created are ideal for growth of bacteria and can lead to bacterial contamination and plugging of the well. Another problem is that cavitation effects can cause the pump to fail prematurely. All of these problems resulting from falling water in the well can cause its capacity to decrease, often alarmingly.